A diffraction grating, when used in a laser cavity as a wavelength selecting device, is usually mounted in the cavity in a Littrow mounting in which the beam diffracted by the grating is collinear with the incident beam. The grating mounted in the cavity provides a wavelength selective feedback which stimulates, in the excited medium of the laser, emission of radiation at the desired wavelength.
The spectral bandwidth of the output laser beam is determined by the passive bandwidth (single-pass bandwidth) of the cavity and the number of light passes in the cavity during the optical pumping of the active medium. A laser cavity defined by a Littrow-mounted grating on one side and a reflector on the other side has a passive bandwidth given by ##EQU1## where .delta..theta. is the half angle divergence of the beam incident on the grating, and d.delta./d.lambda. is the angular dispersion of the beam returning from the grating towards the excited medium. This angular dispersion is given in Principles of Optics by M. Born and E. Wolf (Pergamon Press, Oxford, 1975) as ##EQU2## where m is the diffraction order, a is the groove-spacing of the grating and .theta. is the angle between the diffracted beam (which, in this case, is collinear with the incident beam) and the normal to the grating surface.
The highest wavelength selectivity obtainable with a given grating is achieved when all of its grooves are illuminated by a diffraction limited beam. In many lasers the beam travelling inside the cavity is very narrow so that only a small portion of the grating is illuminated; as a result, the linewidth of the output laser beam is large. The method commonly used to reduce the passive bandwidth of the cavity and thereby improve the laser linewidth is to expand the beam incident on the grating. In this way, the divergence .delta..theta. is reduced and the number of illuminated grooves on the grating is increased. The most widely used method of intracavity beam expansion is with the aid of a lens-telescope as in the dye laser described by Hansch in Applied Optics, volume 11, page 895 (1972), or in the CO.sub.2 laser described by Bagratashvili et al in Optics Communications, volume 9, page 135 (1973). The selectivity of the cavity is thus improved, and a narrower linewidth is obtained; but the use of a lens-telescope inside the cavity has disadvantages including the following:
1. The laser efficiency is reduced due to losses caused by undesirable reflections from the lenses. PA0 2. The alignment of the telescope in the cavity is difficult. PA0 3. The telescope needs focusing from time to time. PA0 4. The beam quality is poor because of the need for using, in the cavity, lenses having a small radius of curvature. PA0 5. The telescope adds significantly to the cavity's length. PA0 6. The telescope has a fixed magnification, so that bandwidth variation is not possible. PA0 7. The illumination of a large area on the grating's surface requires an extremely precise rotatory mechanism for keeping the grating grooves orthogonal to the laser axis as the grating rotates. PA0 8. The high quality telescope required is very expensive.
In order to overcome these difficulties, considerable effort has been expended in replacing the lens-telescope by other types of beam expanders. Two dye lasers with a prism-expander in the cavity are described in the works of Stokes et al in Optics Communications, volume 5, page 267 (1972) and Hanna et al in Optical and Quantum Electronics, volume 7, page 115 (1975). A CO.sub.2 laser with a similar prism-expander is also reported by Alcock et al in Applied Physics Letters, volume 23, page 562 (1973). Another dye laser with a mirror-telescope in the cavity is reported by Eesley and Levenson in IEEE Journal of Quantum Electronics QE-12, page 440 (1976). A multiple-prism-expander for use in a dye laser cavity is reported by Novikov and Tertyshnik in Soviet Journal of Quantum Electronics, volume 5, page 848 (1975) [Kvant. Elektron. (Moscow) volume 2, page 1566 (1975)] and disclosed recently in the U.S. Pat. No. 4,016,504 to Klauminzer (1977). These alternative beam expanders have been used with varying degrees of success but none of them could overcome all the disadvantages of the lens-telescope.
In another prior art dye laser described by Bjorkholm et al in Optics Communications, volume 4, page 283 (1971) a mirror-grating combination is used with the mirror mounted between the dye cell and the grating, but the linewidth obtained is poor.
It has been known for a long time that the wavelength selectivity of a diffraction grating may be improved by using a multiple-pass design. In the work of Hulthen and Lind in Arkiv Fysik, volume 2, page 253 (1950) a description is given of a spectrometer which utilises a combination of a grating and a plane mirror which reflects the beam diffracted by the grating back along its incidence path for a second diffraction by the same grating. The beam which travels away from the grating has, after the second diffraction, an angular dispersion of ##EQU3## which is doubled as compared with Eq. (2). .theta. is the angle between the incident beam and the normal to the grating surface. This combination of grating and mirror has been used also in lasers for the same purpose of doubling the angular dispersion obtained.
Equations (2) and (3) express the well known feature of diffraction gratings that the higher the diffraction angle .theta. the higher is the angular dispersion obtained and the higher is the wavelength selectivity. However, the use of a diffraction grating in a Littrow mounting or in the arrangement described by Hulthen and Lind at an angle of incidence near grazing, e.g. 90.degree. with respect to an axis normal to the surface of the grating, is not, insofar as is known, shown in the prior art. The use of a grating at angles of incidence exceeding 80.degree. has been thought to be impractical because of the low diffraction efficiency and the high reflection losses at these angles.
X-ray spectroscopy is, insofar as is known, the only field in which diffraction gratings are presently used at grazing incidence angles, this being for reasons which are particular to x-rays, e.g. Encyclopedia of Physics, S. Flugge editor (Springer, Berlin, 1967) Volume 29, page 435. The grazing incidence diffraction of x-rays by optical gratings makes use of the fact that the refractive index of materials is generally slightly smaller than unity for x-rays, so that total reflection of x-rays occurs in the vicinity of grazing incidence. In fact, grazing incidence diffraction is the only known means for obtaining spectra at x-ray wavelengths. Another important difference between the diffraction of x-rays and the diffraction of light in grating tuned lasers in that in x-ray diffraction the incident beam and the diffracted beam are both at angles near grazing, but on opposite sides of the normal to the grating.